In recent years, the activity in solar cell research has considerably increased due to the need for solar-to-electricity and solar-to-fuel conversion with increased efficiency. TiO2 is widely recognized as one of the most promising photocatalysts for solar energy utilization, but its energy conversion efficiency has been severely limited by its intrinsic band gap of 3.2 eV, which can be excited only by ultraviolet light (small fraction of the sunlight). Researchers have employed various strategies to make TiO2 more responsive to visible light, including incorporating a dye or sensitizer on TiO2 particles or films and also doping TiO2.
Dye-sensitized solar cells (Gratzel cells) based on TiO2 have emerged as a viable technology for photovoltaics, which have traditionally relied on solid-state junction devices based on silicon. These hybrid organic-inorganic devices separate the two tasks of light absorption and charge carrier transport. An organic sensitizer or dye attached to the surface of a wide band gap semiconductor (e.g., TiO2) absorbs light, and charge separation occurs at the interface by photo-induced electron injection from the dye into the conduction band of the semiconductor. Charge carriers move from the conduction band to the charge collector, and conversion of incident photons into electric current may be achieved over a large spectral range extending from the ultraviolet to the near infrared region. The use of dyes having a broad absorption band in conjunction with high surface area TiO2 films allows a large fraction of sunlight to be harvested. Values exceeding 11% efficiency using mesoporous anatase TiO2 at one sun illumination were achieved by Gratzel in 2006.
A downside of dye-sensitized solar cell technology, however, is that the dye must be regenerated with electrons, and the typical approach involves using a redox electrolyte that includes corrosive media in an organic solvent. Over time, the electrodes may become corroded as a consequence of the regeneration process and may degrade, thereby diminishing the performance of the solar cell. Accordingly, improved technologies are needed to avoid long-term stability problems as well as sealing issues with dye-sensitized TiO2 cells.